The tension on a vehicle seat belt can be used to characterize the seat occupant for purposes of determining whether air bags or other restraints should be deployed in a severe crash event. See, for example, the U.S. Pat. Nos. 6,554,318; 6,605,877; 6,796,192; and 6,851,503, assigned to Delphi Technologies, Inc., and incorporated herein by reference. In a typical mechanization, the seat belt passes through a slot in an armature such that the armature is displaced against a spring bias in relation to the belt tension. A switch may be used to detect belt tension in excess of a threshold; or a magnetic sensor can be used to measure the armature displacement, as described in the above-mentioned patents.